This disclosure relates generally to medical diagnostic imaging methods and systems, and more particularly to medical diagnostic imaging methods and systems that acquire and process tissue information for measuring the liver fat levels of an individual.
Non-alcoholic fatty liver disease (NAFLD) has become a common form of liver disease. It occurs when fat is accumulated in liver without significant alcohol consumption. It is associated with insulin resistance, cardiovascular disease, and metabolic syndrome, such as obesity, combined hyperlipidemia, diabetes mellitus (type II), and high blood pressure. The prevalence of NAFLD in the United States is estimated to be between 10% and 33%. Accumulation of fat in liver can lead to steatohepatitis and cirrhosis, and thus liver fat level may be used as a parameter to quantify the severity of NAFLD. Liver biopsy is considered the gold standard for liver fat diagnosis. But it is costly, invasive, and may be subject to sampling error. The non-invasive imaging modalities for liver fat quantification include MRI (magnetic resonance imagining), proton magnetic spectroscopy (1H MRS or simply MRS), ultrasound and CT scan (computed tomography scan). MRS has been considered the gold standard for in vivo quantification of liver fat. It allows the detection of low level liver fat. However, it is not widely available and requires long examination time.
Fat is more echogenic in ultrasound imaging, and thus liver with a higher fat level is brighter in the image. The sensitivity of ultrasound for liver fat quantification is acceptable, but its capability in grading the degree of liver fat level is limited. Furthermore, ultrasound is unable to distinguish brightness in liver caused by high liver fat level and by high fibrous level. CT has also been studied to estimate liver fat level. In CT imaging, low attenuation indicates low tissue density. Therefore, lower mean liver attenuation in the CT image indicates a higher liver fat level. However, there is risk in CT imaging due to high dose of ionizing radiation in CT imaging and these examinations cannot be conducted frequently. Furthermore, CT imaging is costly. 2D dual energy X-ray absorptiometry has the capability to distinguish the difference between fat and fibrous tissues better than ultrasound, but with a significantly lower x-ray dose and cost than CT imaging. However, due to the 2-D nature of the imaging it is difficult to distinguish between subcutaneous fat, visceral fat and fat in the liver.
Therefore, there is a need for a method and system to accurately measure liver fat levels that can be conducted frequently during routine exams. More specifically, there is a need for a 2-D dual energy x-ray absorptiometry method and system to more accurately measure liver fat by measuring and correcting for subcutaneous fat and other visceral fat.